Add embedded monitoring with simple-jmx-exporter-agent

**Embedded monitoring with simple-jmx-exporter-agent**

* `simple-jmx-exporter-agent.jar` is added as a javaagent (see `$control_dir/java-opts-60-simple-jmx-exporter-agent`)
* MBeans to collect are defined in `tomcat-metrics.xml`
* for backward compatibility, metrics are outputted in a file (`$log_dir/tomcat7-metrics.data`, configurable) which is collected by `stats-appstat` script (with a simple `cat` command)

**Pending tasks**
* verify that the unit of durations metrics returned by `stats-appstat` is mills and not seconds (e.g. `threads.cputime`)
* release simple-jmx-exporter-agent.jar and update its URL in `MakeFile`